"The corridors and seats will be equipped with sway detectors. Anyone who thinks about swaying will be kicked out."

Another tweeted: "No dancing or swaying in a concert! It's like putting ice under the sun and asking it not to melt."

The kingdom's entertainment authority did not respond to AFP's request for comment about the instruction.

With its new modernisation drive, the ultra-conservative kingdom has hosted a series of concerts in recent months by artists such as Lebanon's Hiba Tawaji and legendary Greek composer Yanni.

Men and women are often seen breaking into dance at such events, in scenes that were unimaginable not long ago.

Crown Prince Mohammed bin Salman, who is leading the reform drive, is seeking to balance unpopular subsidy cuts in an era of low oil prices with more entertainment options -- despite opposition from Muslim religious hardliners.

The reform stems partly from an economic motive to boost domestic spending on entertainment as the kingdom reels from a protracted slump in oil prices.

Saudis currently splurge billions of dollars annually to see movie shows and visit amusement parks in neighbouring tourist hubs like Dubai.